1. Field of the Invention
The present invention relates to electromagnetic wave sensors. More specifically, the present invention relates to systems and methods for protecting millimeter wave imaging arrays from infrared and visible light emissions.
2. Description of the Related Art
Millimeter wave (MMW) frequency bands lie in the range from approximately 30 GHz to over one terahertz. This band of frequencies is of particular interest in imaging applications because atmospheric xe2x80x98windowsxe2x80x99 exists through which certain bands of MMW frequencies pass with low attenuation. In particular, the bands near 95 GHz, 140 GHz, 220 GHz, and 330 GHz exhibit low attenuation characteristics even in the presence of air, clouds, fog, rain, and other precipitation. This affords an option for imaging where other frequencies, such as visible spectrum and infrared, are attenuated to such a severe degree that imaging is not practicable. Applications in aerospace, military and commercial exists which gain significant advantage through the use of MMW imaging.
The technology to image in the MMW band is well developed. For example, the assignee of the present invention has developed MMW imaging technology as demonstrated in co-pending U.S. patent application Ser. No. 09/414,988, filed Oct. 07, 1999, by J. Grinberg et al. and entitled MM-WAVE/IR MONOLITHICALLY INTEGRATED FOCAL PLANE ARRAY, the teachings of which are hereby incorporated by reference.
Current MMW imaging technology typically employs an array of broadband MMW antennas that lie in a focal plane array. A lens and/or reflector system is employed to gather MMW energy over a large aperture and focus the energy onto the array of antenna elements. By analyzing the energy incident upon each element of the array, an image can be generated such that each antenna in the array is a pixel in the image. While there are a number of methods to receive and detect MMW energy, an antenna design which utilizes a bolometer to sense antenna current, and therefor incident signal strength, has been found to be efficient and effective in detecting MMW energy.
Briefly, the antenna current is passed through a low resistance material which fluctuates in temperature in response to current flow therethrough. There are a number of thermal considerations in such a design, however, the technology has developed to the point where sensitive imaging performance can be achieved. The heat from the low resistance material is thermally coupled to the bolometer element. A voltage is applied across the bolometer element and the current flowing there through is measured. Since this current flow is proportional to the heat generated, and the heat generated is proportional to the incident MMW energy, the current through the bolometer is proportional to the incident MMW energy. By sensing the current flow in all of the elements in the array, an image can be created.
One disadvantage of this present MMW imaging technology is that bolometers are sensitive to any source of heat. While there are a number of thermal considerations in the foregoing design to control the affects of other heat sources, such as thermal isolation and thermal stabilization, some sources of heat energy are not readily controllable. In fact, bolometers are frequently used to detect incident infrared energy. It is therefore difficult for a MMW bolometer antenna array sensor to discriminate between MMW energy and infrared energy. The same is true for visible energy as well. Since this limitation is generally known by those skilled in the art, an entity desiring to prevent imaging through use of MMW radiation can effect countermeasures to prevent effective imaging. For example, a powerful infrared source, directed at the aperture of the current technology MMW imaging sensor can effectively xe2x80x98blindxe2x80x99 the sensor from forming images. In fact, if the energy source is powerful enough, the heat rise in the bolometer elements may be so great so as to destroy the sensor.
While band pass and notch filters are available to mitigate the foregoing affects, none are effective against such a direct countermeasure. Similarly, in environments with high levels of infrared or visible energy, the MMW reception capability of such an array may be compromised.
Thus there is a need in the art for a method and apparatus for protecting MMW sensors from out of band emissions originating naturally or produced as a countermeasure to effective MMW imaging.
The need in the art is addressed by the apparatus and methods of the present invention. In one illustrative embodiment, the invention is implemented as a sensor having a housing with an antenna. The antenna has a bolometer portion and the housing has a window positioned adjacent to the antenna. A protective pad is coupled to the window and is aligned with the bolometer. The surface area of the protective pad may be substantially the same as the surface area of the bolometer. The protective pad may be formed from metal.
In another illustrative embodiment, the sensor is a millimeter wave sensor and the protective pad functions to block infrared and visible light, and the pad is formed of metal to a thickness that appears opaque in the infrared band. The pad thickness is in the range of 500 to 2000 angstroms. In a refinement, the pad thickness is approximately 2000 angstroms. In another illustrative embodiment, a plurality of antennas form a focal plane array supported within the housing and a plurality of protective pads are coupled to the window. The pads are aligned with the bolometers in the antennas, so that each of the bolometers is protected.
The present invention provides a method for protecting a sensor from infrared and visible radiation. The method is adapted for use with a sensor disposed in a housing with a window and an antenna supported therein. The antenna is located adjacent to the window and has a bolometer. The method comprises the step of forming a protective pad on a surface of the window at a position located adjacent to the bolometer. In a refinement of this method, the surface area of the protective pad is substantially the same as the surface area of the bolometer. In a further refinement, the protective pad is metal. In a further refinement, the sensor is a millimeter wave sensor and the protective pad functions to block infrared and visible light. The pad is formed of metal to a thickness that appears substantially opaque in the infrared band. In a further refinement, the pad thickness is in the range of 500 to 2000 angstroms. In a further refinement, the pad thickness is approximately 2000 angstroms.